This invention relates to a motor vehicle intermediate steering shaft.
A motor vehicle typically includes a steering column, a steering shaft rotatably supported on the steering column, a steering gear remote from the steering column having a steering assist fluid motor, and an intermediate steering shaft for transferring steering torque from the steering shaft to an input shaft of the steering gear. The intermediate steering shaft usually includes a tubular element, a solid element telescoped into the tubular element and rotatable as a unit therewith, a pair of universal couplings for connecting the tubular and solid elements to respective ones of the steering shaft and the steering gear input shaft, and a relatively soft elastomeric bushing between one of the universal couplings and the corresponding one of the tubular and solid elements for interrupting the transfer of torsional vibrations from the steering gear to the steering shaft. When the motor vehicle is driven, the elastomeric bushing twists back and forth and transfers relatively light steering torque, e.g. about 30-40 Nm, between the steering shaft and the intermediate steering shaft because the steering assist fluid motor provides most of the effort necessary to steer the motor vehicle. An emergency coupling parallel to the elastomeric bushing becomes active only if the elastomeric bushing is disabled to assure continuity of torque transfer through the intermediate steering shaft. Such intermediate steering shafts are not suitable for an electric power steering application in which manual steering effort is supplemented by the output torque of an electric motor applied to the steering shaft ahead of the intermediate steering shaft. In such an application, steering torque transferred through the intermediate steering shaft is high, e.g. about 100 Nm, and would repeatedly twist the elastomeric bushing beyond the aforesaid relatively small angular range and thereby destroy its structural integrity. In that circumstance, while the emergency coupling would continue to assure steering torque transfer through the intermediate steering shaft, the corresponding relative angular movement between the steering shaft and the steering gear input shaft would not be commercially acceptable. Simply increasing the torsional stiffness of the elastomeric bushing is not an attractive option because a stiffer bushing would increase the transfer of torsional vibrations from the steering gear to the steering shaft. An intermediate steering shaft according to this invention is a novel improvement over such conventional intermediate steering shafts.
This invention is a new and improved motor vehicle intermediate steering shaft including a tubular shaft element, a solid shaft element telescoped into the tubular shaft element and rotatable as a unit therewith, a pair of universal couplings for connecting the tubular and solid shaft elements to respective ones of a steering shaft on a steering column and an input shaft of a steering gear, and a torsionally elastic coupling between the solid shaft element and the corresponding one of the universal couplings. The torsionally elastic coupling includes a torsion sleeve rigidly attached to the solid shaft element, a bushing cavity around the torsion sleeve in a yoke of the universal coupling, and a relatively soft elastomeric bushing on the torsion sleeve interference fitted in the bushing cavity. A pair of long radial lugs on the torsion sleeve are disposed in respective ones of a pair of notches in the universal coupling yoke with separation therebetween. The elastomeric bushing is relatively soft for maximum torsional vibration isolation at high vehicle speed. At low vehicle speed, the elastomeric bushing twists until the long radial lugs on the torsion sleeve engage the notches in the yoke to rigidly couple the solid shaft element to the yoke parallel to the elastomeric bushing. The long lugs and the notches thus cooperate to prevent the elastomeric bushing from twisting beyond an angle consistent with maximum durability of the bushing.